Methods for producing gasoline boiling range aromatic hydrocarbons from nonaromatic hydrocarbon feeds by employing medium pore size zeolite type catalysts are generally known, as exemplified in, e.g., U.S. Pat. Nos. 3,760,024, 3,843,741 and 4,350,835. In such processes, the desired end product comprises primarily gasoline boiling range materials. The basic reaction is an aromatization reaction. Gasoline, as such term is used herein, and as such term is commonly used in the petroleum industry, is useful as a motor fuel for internal combustion engines. More specifically, gasoline is hydrocarbon in nature, and contains various aliphatic and aromatic hydrocarbons having a full boiling range of about 280.degree. to 430.degree. F., depending on the exact blend used and the time of year. Although gasoline is predominantly hydrocarbon in nature, various additives which are not necessarily exclusively hydrocarbon are often included. Additives of this type are usually present in very small proportions, e.g., less than 1% by volume of the total gasoline. Further, it is also not uncommon for various gasolines to be formulated with non-hydrocarbon components, particularly alcohols and/or ethers as significant, although not major, constituents thereof. Such alcohols, ethers and the like have burning qualities in internal combustion engines which are similar to those of hydrocarbons in the gasoline boiling range. For purposes of this specification and the present invention, however, the term "gasoline" denotes a mixture of hydrocarbons boiling in the aforementioned gasoline boiling range and is not intended to include the above-referred to additives and/or non-hydrocarbon constituents.
High octane gasoline is desirable for use with internal combustion engines from a standpoint of fuel efficiency, and thus is also attractive from an economic perspective. Further, the gradual phasing out of lead in gasoline has created a demand for new methods for obtaining high octane gasoline. It is known that aromatic gasoline boiling range hydrocarbons have high octane (R+O), (M+O) and/or (R+M)/2 values. It is known that gasoline octane is related to the aromatic selectivity of the catalyst employed in the reforming process used to produce gasoline boiling range hydrocarbons. An increase in aromatic selectivity will result in increased gasoline octane. Aromatic selectivity, as used throughout is defined as (wt. % aromatics produced)/(100-wt. % C.sub.2 =+) where C.sub.2 =+ is ethylene and high paraffins and olefins in the product.
Processes for converting paraffinic hydrocarbons to aromatics using a single conversion zone or reactor containing a noble metal/low acidity medium pore size zeolite catalyst are generally known, as are processes using a single conversion zone or reactor containing a medium pore size acidic zeolite catalyst, which may contain a dehydrogenation metal. Also, aromatization processes using two conversion zones containing the same or different catalysts in the same reactor or separate reactors are also known.
For economic reasons, there is a clear need to increase aromatic selectivity of catalysts employed in processes used to produce aromatics. Hence, methods which are capable of increasing aromatic selectivity of the catalyst are very desirable.